In the context of oil extraction, oil reservoirs can be fractured, thereby trapping oil even upon completion of classical oil recovery processes, such as water flooding, gas injection and in-situ combustion. There are known processes, sometimes referred to as “secondary” and “tertiary” oil recovery, aimed at recovering portions of this trapped oil. One such process is polymer flooding. This is a flooding process that uses natural polymers, such as xanthan and starch, and synthetic polymers, for example, polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM). The polymer can increase the viscosity of the injected water. This can increase sweep efficiency due to the improved mobility ratio, and can reduce the total volume of water required to reach the residual oil saturation. Use of PAM and HPAM, identified above, are known in the field of polymer flooding, generally for controlling the mobility ratio of water to oil. PAM and HPAM can increase the water viscosity, which hinders a phenomenon known as “water fingering” and, instead, operates to push the oil forward or to the front for retrieval.
The above-described operations of PAM and HPAM result from the high molecular weight of the polymers and the repulsion between the negative charges along the polymer chain of HPAM, each being based on maximum chain extension. A limitation of traditional polymer flooding, however, is that the polymer chains exhibit degradations during the flooding process. Examples include thermal degradation, as well as mechanical, salinity, shear and biological degradations of polymer chains during the flooding process.
In addition, shearing and heating in wellbores and reservoirs often decrease the polymer viscosity. Polymer solutions, as any non-Newtonian fluid, conform to the power law, including viscosity, shear rate, consistency index, and flow behavior index. The polymer solution viscosity is thus easily calculated under any shear rate based on the determined consistency and flow behavior indexes. As is understood in the art, the consistency index increases with increasing polymer concentration, but decreases with increasing temperature. On the other hand, the flow behavior index decreases with increasing polymer concentration, and slightly increases at high temperature. Therefore, it should be understood that a higher HPAM concentration leads to higher viscosities, and polymer viscosity is reduced at a higher shear rate and temperature.